Currently titanium oxide-based sunlight energized photo catalysis is the most efficient, yet poorly understood conversion process for hydrogen generation by water splitting using solar energy.  Making use of solar to process water to hydrogen is considered a major challenge and goal because it offers a direct sun to fuel solution.

Scientists of the KIT Institute for Functional Interfaces collaborating with colleagues from the universities of St. Andrews (Scotland) and Bochum and Helmholtz-Forschungszentrum Berlin, Germany have studied the basic mechanisms of photochemistry of titanium compunds and have presented new detailed findings in Physical Review Letters entitled Photocatalytic Activity of Bulk TiO_{2} Anatase and Rutile Single Crystals Using Infrared Absorption Spectroscopy.

Scientists at KIT’s Institute for Functional Interfaces (IFG), headed by Professor Christof Wöll, have succeeded in gathering new findings on the fundamental mechanisms of photochemistry on titanium dioxide (TiO2).  Hydrogen production from water and sunlight by means of oxide powders has been studied extensively for several decades, but the basic physical and chemical mechanisms of the processes involved cannot yet be described in a satisfactory way.  The team has closed up the gap in understanding.

Titanium dioxide, also known as titania, is a photoactive material that occurs in nature in two forms, the rutile and anatase.  Anatase has been characterized by a ten times higher photochemical activity. When the white TiO2 powder, which is familiar because its also used as a pigment in paints and sunscreens, is exposed to light, electrons are excited and can, for example, split water into its components oxygen and hydrogen.  The hydrogen that’s produced in this process is a “clean” energy source as no chemicals are generated or emitted.  Only water is produced during the hydrogen’s re-combustion with oxygen.

Titanium dioxide is also used to manufacture self-cleaning surfaces from which unwanted films are removed through photochemical processes triggered by incident sunlight. In hospitals, this effect is used for sterilizing specially coated instruments by means of UV irradiation. It’s really handy stuff.  Now you know why so much medical equipment is made with titanium, too.

Titania Molecule Forms Rutile and Anatase. Click image for more info.

The explanation of the physical mechanisms of these photochemical reactions on titania surfaces and especially the reason for the much higher activity of anatase has come over the threshold of understanding.  The powder particles used in photoreactors are as tiny as only few nanometers, thus too small for use in studies by means of the powerful methods of surface analysis. Instead the team used millimeter sized single-crystal substrates, making for the first time an ability to precisely study photochemical processes on the surface of titanium dioxide by means of a novel infrared spectrometer.  Using a laser-based technique, the scientists determined the lifetime of light-induced electronic excitations inside the TiO2 crystals.

Professor Christof Wöll, Head of the IFG, explained that exact information about these processes is of great importance, “A short lifetime means that the excited electrons fall back again at once. We witness some kind of an internal short circuit. In the case of a long lifetime, the electrons remain in the excited state long enough to be able to reach the surface of the crystal and to induce chemical processes. Anatase is particularly well suited for the latter purpose because it is provided with a special electronic structure that prevents ‘internal short circuits’.”

Professor Olaf Deutschmann, spokesman of the Helmholtz Research Training Group on Energy-related Catalysis points out knowledge of this feature will allow the team and other researchers to further optimize shape, size, and doping of anatase particles used inside photoreactors. The objective is to develop photoactive materials with higher efficiencies and longer lifetimes “The results obtained by Professor Wöll and his co-workers are of great importance regarding the generation of electrical and chemical energy from sunlight, and especially regarding the optimization of photoreactors,” Professor Deurschmann said.

The Europeans have a good idea here with some working results. This is the kind of thing engineers need to know to design and improve process and the materials used.  Know how on water splitting has been pretty thin, but there is help in the team’s paper and a route to test other ideas.


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